In general, full advantage of lithography and etching technologies is taken in the field of electronic devices such as ultra LSIs or magnetic devices, and these devices are fabricated by combining these techniques.
The etching technique is a technique for fabricating a device which comprises transferring a resist pattern produced by lithography onto an object to be processed, i.e., to a semiconductor thin film, a magnetic thin film, etc., and includes methods such as wet-chemical etching method, argon ion milling method, and reactive ion etching method. Among these etching methods, reactive ion etching method is a kind of dry etching method, and is advantageous in that it enables a most precise transfer of patterns produced by lithography, and that it is suitable for fine processing. Moreover, it boasts superior etching rate. In view of such advantages, numerous large integrated circuits and semiconductor memories are fabricated by the reactive-ion etching method.
The reactive-ion etching method comprises placing the work piece in a plasma of a reactive gas while applying an electric field thereto, and physically and chemically stripping off successive layers of atoms by the incident ion beams that are irradiated vertically to the surface of the work piece. This method enables anisotropic processing cutting vertically along the boundary of the mask, and hence, it allows transfer of fine and sharp patterns.
In case of reactive-ion etching, firstly, the chemically active species such as the ions or radicals of the reactive gases that are generated in the plasma are adsorbed onto the surface of the work piece and undergo chemical reaction to form a layer of chemical products having a low bonding energy. Since the surface of the work piece are exposed to the impact of the positive ions that are accelerated in the plasma by an electric field and which are vertically incident to the surface, the surface layers that are loosely bonded are successively stripped off by the sputtering of ions or by the evaporation into vacuum. In this context, the reactive-ion etching process can be regarded as a process in which a chemical reaction and a physical process proceed simultaneously, and it is characterized by having a selectivity on a specific substance and having anisotropy as such to cut vertically into the surface of the object.
However, despite the superiority of the reactive-ion etching method over other methods, no effective means has been found for etching copper or gold that are widely used in the electronics, or for silver that is used in abundance as a heat conductive material or an electric contact material. The reason for this is that copper, silver, and gold undergo reaction with various types of etching gases such as CF4, CCl4, CCl2F2, CClF3, CBrF3, Cl2, C2F6, C3F8, C4F10, CHF3, C2H2, SF6, SiF4, BCl3, PCl3, SiCl4, HCl, CHClF2, etc., which are developed for etching semiconductor materials, and form reaction products with a bonding energy far higher than semiconducting materials. Thus, the reaction products are less apt to be subjected to a sputtering or an evaporation, and cannot be removed in a plasma.
Under the aforementioned circumstances, wet-chemical etching process or argon ion milling process has been conventionally applied to copper, silver, and gold to fabricate, for instance, a thin film magnetic head, a magnetic sensor, a micro transformer, etc. Furthermore, aluminum has been used for the electrodes and interconnections necessary for semiconductor devices by taking advantage of the ease in applying reactive-ion etching process at the expense of a high electric resistance and a high heat emission.